(1) Field of the Invention
This invention relates to environmental control systems for controlling the temperature of a space within a structure and, more particularly, to a method practiced by the programmable thermostat of such a system to utilize the thermal characteristics of the structure to increase the energy efficiency of the system.
(2) Description of Related Art
Heating and air-conditioning, or environmental control, systems function to maintain the interior temperature of a defined space within a structure at a particular nominal, or set point, temperature. Control by such a system is accomplished by the use of a thermostat which senses the temperature in the space within the structure. As the temperature deviates from a set point temperature, the thermostat activates the heating or cooling plants of the system to bring the temperature sensed by the thermostat back toward the set point temperature.
Currently, available programmable thermostats include a microprocessor, a random access memory (RAM), a read-only memory (ROM), and a clock. The microprocessors of such thermostats are programmed to perform, among other things, temperature set point changes at preprogrammed times. For example, when the system is in its heating mode, the current occupied set point temperature can be set back to an unoccupied, or lower, setback temperature at a programmed setback time to conserve energy while the space is unoccupied. When the system is in its cooling mode, the current occupied set point temperature can be advanced, or setup, to the unoccupied higher setup temperature at a programmed setup time.
The problem with existing programmable thermostats, particularly those used to control the temperature in a home, for example, is that they operate independently of the thermal characteristics of the structure of the home and the environment in which the structure is located. They do not take advantage of the fact that when heat, for example, is not supplied to a home that a certain amount of time is required before the temperature of the home begins to move toward the unoccupied temperature. This thermal lag is a result of the thermal mass of the home and its contents, and the current external environment of the home which includes air temperature, wind velocity and direction, and incident solar radiation. Programmable thermostats typically change the set point temperature to the unoccupied set point temperature at the beginning of the unoccupied period, the unoccupied set point time. By not taking advantage of the thermal lag, a structure exhibits vibration, noise and the sudden increase in air flow frequently associated with turning on the heating or cooling units of the system shortly before the thermostat changes its set point temperature from its occupied to unoccupied values. Further, by not utilizing the stored energy available from the thermal lag of the structure, all of the possible energy saving is not achieved.